The RF (radio or high frequency) power transistor is today a mature product that provides solid-state sources in kilowatt FM broadcast transmitters, TV transmitters, two-way mobile radios, cellular telephones, aircraft communications, radar, and a variety of military applications. Power levels to kilowatts attest to the advances in this area of the semiconductor industry in the last 25 years. Higher power and higher frequency operation are much desired.
The use of semiconductor devices for RF applications has increased dramatically as heat dissipation, size, dependability, and other improved characteristics have made such devices well-suited for many applications. Semiconductor transistors, including an emitter, a base, and a collector as the three, main components, are small; therefore, they are generally incorporated into packages. RF packages are used to hold semiconductor components, particularly transistors, and to provide readily available terminals for connection to other components.
In designing such packages, thermal management is critical--especially at high power generation levels which increase the amount of heat created. Heat decreases the performance and operating life of the transistor. Moreover, heat generated at the collector-base junction can create a temperature buildup which may eventually cause the device to short-circuit.
RF transistors often require, therefore, water-cooled assemblies to carry away the heat. Alternatively, the transistor may be mounted on a thermally conductive pad which acts as a heat sink and dissipates the heat generated by the transistor. In order to prevent electrical losses, however, the thermally conductive pad must also be electrically insulating. Hence, a thermally conducting, electrically insulating material is used for the mounting pad. Typically, that material is beryllia (beryllium oxide--BeO), although alumina (aluminum oxide--Al.sub.2 O.sub.3) and, more recently, boron nitride (BN) have occasionally been used.
Beryllia (BeO) has better thermal conductivity than Al.sub.2 O.sub.3 and, consequently, is more commonly used. Care must be taken in handling and processing BeO, however, because BeO is highly toxic. It can be hazardous to the human respiratory system when in the powder form created as the BeO pad is machined or ground. Proper equipment and safeguards are needed to insure that the BeO is handled safely and that improper contact with humans is avoided. The military has prohibited the use of BeO except under exceptional circumstances. There is a government-funded initiative to replace BeO with BN.
Another disadvantage of the prior art packages is that, when BeO is a component, a "thin" pad of BeO is used. The pad is attached to the heat sink by a preform, which creates two interfaces: pad-to-preform and preform-to-heat sink. These interfaces decrease the thermal conductivity of the connection and require extra processing steps.
Further, although BeO has a high thermal conductivity up to about 200.degree. C. to 250.degree. C., its performance decreases with increasing temperature. Attempts to drive the RF transistor past the normal operating level to higher power levels will fail, therefore, because the BeO pad becomes thermally saturated and unable to dissipate the heat generated. When heat dissipation fails, the RF transistor will slump and its power will simply drop off. Such a slump is typical of existing RF transistor packages.
To improve thermal management, diamond spreaders have been used. Diamond is an excellent thermal conductor which can be incorporated in the RF transistor package. A package using diamond will dissipate heat much faster than BeO and can achieve higher power levels without slumping. Diamond is, however, an expensive material. Its use indicates the importance of thermal management in high power RF transistor packages. The Be0 pad used is generally about 1 to 1.5 mm thick. Such thickness is required because the mechanical strength of BeO is low. If insufficiently thick, the pad may crack or split either during attachment or later upon heating during use. Thus, the overall height of the RF package is increased when a BeO pad is used.
Most commonly, RF transistors operating in high power and frequency ranges are fabricated from several semiconductor die operating in parallel within a single housing or assembly. Large numbers of complex internal parts having high cost are used which require an excessive number of welded or soldered connections adversely affecting reliability. It would be desirable to create monolithic circuits in which the semiconductor die are operating on the same piece of substrate material.
RF transistors also typically have one or more top surface contacts and at least one bottom surface contact. Thus, a multi-plane terminal arrangement exists. Such an arrangement is troublesome because it requires more internal interconnections, increases lead length, and raises inductance. Parasitic couplings between input and output circuits cause losses and unwanted feedback effects which degrade performance. Single, top, planar surface contacts are desirable.
To improve upon the existing RF transistor packages, a new, bipolar, monolithic, high-power RF transistor with isolated top collector contacts is provided. An object of the present invention is to provide an improved transistor which operates efficiently and reliably even in high power and high frequency ranges. A related object is to improve thermal management in the RF transistor package, dissipating heat as rapidly as it is generated, by providing vertical isolation. Upon achieving these related objects, power slumps will be avoided.
Another object is to replace the typical thermally conducting, electrically insulating BeO mounting pad and, thereby, to avoid the problems inherent in using such a pad. Such replacement is preferably done without incorporating expensive materials such as diamond. Further objects are to use conventional planar processing steps and equipment, and to simplify those processing steps required, in manufacturing the improved RF transistor package. Simplification is possible, for example, by avoiding the typical step of attaching the BeO pad to the heat sink.
It is still another object of the present invention to provide all contacts, including an isolated collector, on the top surface of the package. Yet another object is to build an RF transistor on an intrinsic, electrically insulating substrate so that the transistor die can be mounted directly to the heat sink. A related object is to make the RF transistor into a monolithic circuit by laterally isolating the RF transistor; different electrical components can be placed on the same piece of substrate material. Such lateral isolation also solves the parasitic capacitance problem which is especially pronounced in high power RF transistors.